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400 BC Greece-philosophers Democritus said that there must be something that cannot be divided He called this particle atomos Aristotle claimed that no such particle exists Neither did any experiments to test their claim They were just thinkers
Definition: An atom is the smallest particle into which an element can be divided and still be that element. Considered the basic unit of matter even though it is made up of certain other particles
John Dalton-1808 Used experimental evidence to make a few major conclusions All mater is made up of atoms All atoms of a certain element are identical Every substance is made up of atoms combined in certain ways
JJ Thomson-1897 Used evidence to explain that there a particles of negative charge mixed through Dalton’s original sphere Ernest Rutherford-1909 Discovered a nucleus (tiny dense positive center) Positively charged particles called protons The atom is mostly empty space
Niels Bohr Electrons move in circular paths around the nucleus Each path is a certain distance from the nucleus and has a certain amount of energy associated with it James Chadwick Discovered the nucleus had uncharged particles called neutrons
Erwin Schrodinger Electrons move around the nucleus not in circular orbits but instead in areas called the electron cloud
Protons Positively charged particles (1+) It has a mass of 1.7 x grams This is equal to a mass of 1 amu or u Neutrons No electrical charge (neutral) Slightly larger than protons but about 1 amu
Negatively charged particles (1-) located outside the nucleus So small they are about 0 amu It is not possible to find their exact positions and speed at the same time (this is why we call it a cloud) The number of electrons in an atom equals the number of protons giving it a net charge of 0 When an atom gains or loses electrons we call the atom an ion
Atomic number The number of protons in an atom Mass Number The total number of protons in neutrons in an atom Atoms can have different number of neutrons so we use calculations to find an average mass number